Display User ID

You can edit the emails via the menu System->Emails, there you can add something like this:
<?php echo "Your id: ".$data->order_user_id; ?>

You can edit the file form of the view order via the menu Display->Views and add something like this:
<?php echo "User id: ".$this->order->order_user_id; ?>
It's strange that you're asking because this id is already displayed in the order details page on the back end...

I've changed the code in my message. Thanks.

Yes. You need to go in your phpmyadmin and change the auto increment value of the jos_hikashop_user table:
ALTER TABLE jos_hikashop_user AUTO_INCREMENT = 1000;

Then, new users will have their ID starting from 1000 and not 1